The present invention relates to a method for stabilizing phosphorus trichloride.
Phosphorous trichloride is prepared industrially by the reaction of phosphorus with chlorine. It generally contains less than 0.05% phosphorus oxychloride, if it is prepared from purified chlorine, or if it is rectified when chlorine is used, which has not been purified.
However, phosphorus trichloride can easily be oxidized by oxygen or oxygen-containing gases to phosphorus oxychloride. A slow, steady increase in the phosphorus oxychloride content can be observed during storage, transport and use of phosphorus trichloride. The phosphorus oxychloride lowers the quality of the phosphorus trichloride and necessarily leads to undesirable by-products during chemical processing, so that the desired end product is of lower quality and is obtained in a lesser yield. For example, in further processing to phosphorus pentachloride or phosphorus sulfochloride, all the phosphorus oxychloride reaches the end product. Because of the physical properties, subsequent separation is much more difficult than separation from phosphorus trichloride. During further processing to phosphorous acid, phosphoric acid is formed from the phosphorus oxychloride and phosphates are formed as by-products during the reaction with alcohols to phosphites. During the synthesis of tri-(2-chloroethyl) phosphite by reaction with ethylene oxide, tri-(2-chloroethyl) phosphate is formed as an interfering by-product.
During the synthesis, storage and transport of phosphorus trichloride, care must therefore be taken to exclude air or other oxygen-containing gases as far as possible. The storage of phosphorus trichloride under a slight excess pressure of a dry, oxygen-deficient inert gas is known (Chem. Techn. 33 (1981), vol. 4, page 191). In practice, the synthesis and transport of phosphorus trichloride are also carried out while largely excluding oxygen, if necessary, under the protection of an inert gas, such as nitrogen or carbon dioxide. Nevertheless, during prolonged transport and storage, oxygen has a pronounced effect and there is formation of phosphorus oxychloride. For example, when shipping tank car lots, concentrations of mainly 0.2% to 0.8% phosphorus oxychloride occur.
Because of the high cost of energy and equipment, subsequent or repeated purification by distillation cannot be given consideration.
Decreasing the amount of and stabilizing the phosphorus oxychloride by adsorbents, such as silica gel (Gallus-Olender, I. and Franc, B.; Z. anorgan. allg. Chem. 451 (1979), pages 189-192 and Poland Patent 85 068) are also very disadvantageous because of the large expenditure for equipment, the costs of the adsorbent, the product losses and the disposal of waste. For example, to lower the POCl.sub.3 content from 0.78% to 0.07%, calculated as H.sub.3 PO.sub.4, and to stabilize the phosphorus trichloride, a contact time of 21 days and 10% silica gel, based on the amount of phosphorus trichloride, are required.